Let me take it one step further, though... a user should be able to set their wp.org username on their profile page, so that their favorite plugins are more easily accessible from the add plugins screen.

In my opinion, this is well worth the added server load it will create. I also think this will get many more WordPress users to actually use the excellent favorite feature that already exists. This could not only offer a great solution to an unfinished feature, it can also lead to an alternate way to determine the most popular plugins (how many favorites this year/month/week).